Study terminated due to business and strategic decision

The purpose of the study is to evaluate the efficacy, safety, tolerability and pharmacokinetic (PK) profile of prophylactic SerpinPC in participants with Hemophilia B with inhibitors, as part of the SerpinPC registrational program.

Inclusion Criteria:

1. Male participants greater than or equal to (>=) 12 and less than or equal to (<=) 65 years of age at the time of informed consent.
2. Capable of providing written informed consent (adolescent assent and parental/guardian/legal representative consent when appropriate) for participation and having the opportunity to discuss the study with the Investigator or delegate.
3. Historically documented HemB (defined as factor IX <=0.05 international unit/Milliliter (IU/mL) [<=5 percent (%)]).
4. Participants who are currently in a prophylaxis program must be willing to stop prophylaxis (including episodic prophylaxis for sporting events) before the first dose of SerpinPC.
5. Historical or ongoing Factor IX inhibitor with bypass agents based on medical records or laboratory reports.
6. Documented ABR of 6 in the 12 months before screening (participants not on prophylaxis regimen) or documented ABR of ≥2 for participants on prophylaxis regimen
7. At least 12 weeks of prospective documentation of bleeding episodes in the AP-0105 (NCT05605678) non-interventional study before SerpinPC dosing, or willing to complete a 12-week observational period (at minimum) in AP-0103.
8. No bleeding in the 7 days before Baseline (the prospective observation period can be extended by 10 days if there is an ongoing active bleed).
9. D-dimer of <=750 micrograms/Liter (mc/L); in cases where there is a resolving bleed, the exclusion threshold is <=1750 mg/L at Screening and Pre-dosing visits.
10. Adequate hematologic function, defined as a platelet count of >=100,000/microliters (mcL) (>=100*10^9/L) and hemoglobin level of >=10 grams/deciliter(g/dL) (>=100 g/L or >= 6.206 millimoles per liter (mmol/L) at Screening and Pre-dosing visits.
11. Adequate hepatic function, defined as a total bilirubin level of <=1.5*upper limit of normal (ULN) (excluding Gilbert syndrome) and aspartate aminotransferase and/or alanine aminotransferase of <=3*ULN at Screening and Pre-dosing visits; no clinical signs or known laboratory or radiographic evidence consistent with cirrhosis of the liver.
12. Adequate renal function, defined as a serum creatinine level of <=2.0*ULN at Screening and Pre-dosing visits.
13. Able to use a diary to document bleeding events and medication usage.
14. Sexually active participants with a partner who could become pregnant should agree to use effective contraception for the duration of the study.

Effective contraceptive measure include condom with or without spermicide, a combination of male condom with either cap, diaphragm, or sponge with spermicide (double barrier methods), vasectomy, partner using stable contraceptive measures (combined [ estrogen and progestogen-containing] hormonal contraception or progestogen-only hormonal contraception initiated 2 or more menstrual cycles prior to screening, intrauterine device [IUD]. Intrauterine hormone-releasing system [IUS], bilateral tubal ligation), and/or sexual abstinence.

Exclusion Criteria:

1. Known severe thrombophilia (defined as antithrombin deficiency and/or protein S deficiency and/or protein C deficiency).
2. Participant with previous factor IX inhibitor who responded to immune tolerance induction and remains on prophylactic factor concentrate.
3. Previous deep vein thrombosis, pulmonary embolism, myocardial infarction, or stroke.
4. History of intolerance to SC injections.
5. Uncontrolled hypertension (systolic blood pressure >160 millimeter of mercury (mm Hg); diastolic blood pressure >100 mm Hg).
6. Weight >150 kilograms (kg) OR body mass index >40 kg/meter square (m^2).
7. Has active cancer and/or requires therapy for cancer, except for basal cell carcinoma.
8. Participation in another clinical trial (except for AP-0105 [NCT05605678]) during the 30 days before screening.
9. Prior, ongoing, or planned treatment with gene therapy for HemB
10. Any major medical, psychological, or psychiatric condition that could cause the participant to be unsuitable for the study or could interfere with the interpretation of the study results.
11. History of or other evidence of recent alcohol or drug abuse as determined by the Investigator (in the 12 months before screening).
12. Known human immunodeficiency virus (HIV) infection with CD4 count (or T-cell count) of <200 cells/mcL within 24 weeks before Screening and Pre-dosing visits. Patients with HIV infection who have CD4 > 200 and meet all other criteria are eligible.
13. Current or planned treatment with anticoagulant or antiplatelet drugs
14. Is planning to donate/bank sperm during SerpinPC treatment AND within 30 days of last dose of SerpinPC.
15. Any other significant conditions or comorbidities that, in the opinion of the Investigator, would make the participant unsuitable for enrollment, or could interfere with participation in, or completion of the study.